Well, you can download WhiteLight GPL sources, build the toolchaine, cross-compile bash, compare with the official FW, add what's missing to your drive - and you have bash without any issues.
Damn, sounds easy, but took me so much time and effort with the BlueRing :D

In general, it's not a good idea to replace root's SHELL - what if the add-on /opt/bin/bash has a bad build?
You should look into:
1) Create a separate user with /opt/bin/bash as shell, optionally give sudo access to that user;
2) In the script that you specifically need bash feature, use #!/opt/bin/bash.